1. Technical Field
This invention relates to photovoltaic systems for converting light to electric energy using multi-cellular panels and more particularly to the maximization of the power delivered by each panel.
2. Description of the Related Art
A multi-cellular panel photovoltaic system for converting light to electric energy can be an isolated system (stand alone) with batteries for accumulating energy for delivering electric power even when the panels are not irradiated. In areas where public distribution mains are available, so-called grid-connected systems are preferably used, capable of delivering the electric energy generated to the public distribution mains through bidirectional meters.
Even if the novel system of this disclosure is equally useful for grid-connected systems as well as for isolated systems, the ensuing description and the functional schemes used for illustrating it will refer to the case of a grid-connected system that, as will become clear, employs dedicated safety devices in addition to the core hardware of the conversion system.
Typically, a grid-connected photovoltaic system has a basic scheme as depicted in FIG. 1:                the photovoltaic panels PV-PANELS are the basic components that convert the light impinging thereon into electric energy;        the block INVERTER comprises a first input DC-DC converter that stabilizes the voltage generated by the panels. Depending on the type of power plant and on the number of panels connected in series, the converter can be either a “boost” converter (step-up), a “buck” converter (step-down) or a “buck-boost” converter;        the second block DC-AC represents the output converter that converts the stabilized DC voltage to an AC voltage for transferring the energy delivered by the photovoltaic panels to the electric mains (at the established voltage and frequency);        the safety device ANTI-ISLANDING CIRCUITRY disconnects the photovoltaic power plant from the mains in case the mains are grounded by accidental cause or for maintenance reasons, for obvious safety reasons;        the block CONTROL SYSTEM comprises one or more programmable controllers MCU-1 and MCU-2 that may be either common DSP devices or microprocessors μP with the function of:        executing an appropriate algorithm for calculating the instantaneous power, for tracking the MPPT and controlling the DC-DC converter;        executing an appropriate algorithm for monitoring and analyzing the mains voltage also for recognizing an eventual significant voltage drop and consequently controlling the DC-AC inverter and mains disconnecting/connecting anti-islanding devices.        
MPPT stands for Maximum Power Point Tracking and the results of this algorithm (of which there are various embodiments) determine consequent control signals of the DC-DC converter such that the working point of the photovoltaic panel or panels be kept in the neighborhood of the point corresponding to the maximum deliverable power.
“Anti-islanding” designates a safety system that disconnects the power plant from the public mains in case of failures. The anti-islanding system is composed of devices (switches) that disconnect the power plant and the relative control system based on an algorithm that continuously processes fluctuations of the mains voltage or variations of specific control signals transmitted through the mains for recognizing events of significant voltage drop that may be due to accidental grounding of mains to a deliberate grounding of mains.
Often a sole DSP device or a sole microprocessor μP of appropriate calculation capability may be used instead of two distinct controllers (this possibility is symbolically indicated in the figure by the bidirectional arrow joining the two blocks MCU1 and MCU2).
The commercially available photovoltaic panels differ among them in terms of fabrication technology, physical structure of the photovoltaic semiconductor junction, materials that determine conversion efficiency, power/area ratio and, obviously, cost.
Only for sake of example, the ensuing description will refer to a domestic power plant of about 3 kW, though the same considerations apply to power plants for different applications and of any different power rating.
In order to cover the power requirements of a household (about 3 kW), and considering the yields of nowadays commonly marketed panels, about twenty panels may be required.
Generally, photovoltaic plants can be realized by connecting the panels according to three different schemes:                “single stage”, wherein all the panels of the plant are connected in series and there is a single electrical regulation and conversion sub-system that carries out both the DC-DC function as well as the DC-AC function;        “dual stage”, wherein all the panels are connected in series to a single DC-DC converter that, on its turn, has a DC-AC connected in cascade;        “multi stage”, wherein the panels are grouped into a plurality of groups of panels, each group of panels being connected to a DC-DC converter and on their turn the outputs of all the distinct DC-DC converters are connected in parallel to the input of a single output inverter DC-AC.        
The cost of the photovoltaic panels remains by far the largest contribution to the total cost of the photovoltaic plant.
A way of reducing the cost of an installation is desirable to reduce the number of panels to be installed though guaranteeing the rated power of the power plant.
This objective may be pursued by enhancing the efficiency of each panel for increasing the power that may be delivered at the same irradiation conditions.
Each panel is characterized by a family of V-I characteristics that illustrate the performances for different irradiation conditions. FIG. 2 reproduces a family of curves of a common photovoltaic panel currently marketed.
FIG. 3 is a graph of the power delivered by the panel in function versus the voltage at its terminals, and the maximum power point (MPP) is highlighted.
In order to maximize the power deliverable by the panel, the system should ensure at any time that the panel(s) work in the neighborhood of the working point of its characteristic at which the delivered power is maximized (MPP).
By connecting in series all the cells of the array to the terminals of the panel, there are potential problems: if one or more cells of a panel are for some reason significantly less illuminated than the other cells, the whole series of panels would be limited to work at the current imposed by the underexposed cells; in the case of failure of one or more cells or of their complete obscuration, interruptions may take place (i.e., an open circuit may occur in the current path of the DC source represented by the panel).
In order to lessen the effects of accidental current limitations and/or interruptions, the array of cells of each panel has one or more bypass diodes that, properly connected between the different nodes of the string of cells in series, provide unimpeded alternative current paths to those across one or more cells that may be severely underexposed (obscured) or damaged. Bypass diodes are usually installed in a so-called “junction box” of the panel that houses the terminals for electrically connecting the panel.
In the example shown in FIG. 4, there are three bypass diodes (D1, D2 and D3) and the cell with a cross sign is supposed to be obscured or damaged. Without the diode D1, the highlighted cell would interrupt the current generated by the string of cells of which is part (and eventually current coming from another panel connected in series through the terminal “−” of the panel). The presence of the other diodes D2 and D3 is irrelevant if the other cells of the panel are exposed to a normal irradiation.
In the “single-stage”, “dual-stage” and “multi-stage” arrangements, connection in series of the panels allows at most (for the “multi-stage” structure) the maximization of the power extracted by a string of panels.
According to the system architectures offered on the market, such a maximization is achieved through an algorithm, known by the acronym MPPT (Maximum Power Point Tracking), usually implemented via software and executed by a programmable device such as a digital signal processor (DSP) or a micro-controller (μP).
Depending on the calculation power, the same device (indifferently a DSP or a μP) may perform a centralized management of the whole system. The control circuits of the DC-DC stabilization converter, of the output DC-AC inverter and eventually of the disconnection/connection devices from the public grid of the “anti-islanding” circuitry may be managed by a single DSP or μP of appropriate calculation capability.
According to known architectures, the DC-DC converter or converters if more than one, the output DC-AC inverter and the eventual connection/disconnection circuitry of the “anti-islanding” system are conveniently housed in a central appliance, normally installed at distance from the photovoltaic panels (often close to the bidirectional counter of the mains operator) that includes also a single or two programmable control blocks, indifferently whether they are DSP or μP and eventually also related programming and/or setting interfaces.
Centralization of the management of working point control for maximum power yield of the photovoltaic plant requires necessarily the use of a central processing unit, indifferently a DSP or a μP, capable of executing the numerous calculations contemplated by a relatively complex control algorithm, in order to monitor continuously the electric power being yielded by the panels and consequently intervening, through digital control signals, on the functioning parameters of the DC-DC converters for regulating the DC voltage being converted to an AC voltage.
Obviously, monitoring of voltages as part of input variables and actuating control adjustments includes input and output interfaces of signals with appropriate A/D and D/A converters.
The cost of such a central control and conversion appliance is not negligible.
Moreover, control of the working point of the photovoltaic cells of the panels for enhancing power yield of a photovoltaic energy conversion of plant as implemented nowadays is an operation the effectiveness of which is only partially exploited.